1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine that includes a variable valve-characteristic mechanism that changes a valve characteristic that includes at least one of the duration and the maximum lift of an intake valve.
2. Description of the Related Art
In commonly-used valve systems for an internal combustion engine, an engine valve is closed by the force of a valve spring. When the engine valve is opened, the engine valve is pushed downward by the cam of a camshaft directly, or through a rocker arm or the like. In such valve systems, a valve characteristic of the engine valve, such as the duration and the maximum lift, is constant, irrespective of the engine operating state.
Recently, a valve-characteristic control apparatus for an internal combustion engine has been proposed. The control apparatus includes a variable valve-characteristic mechanism that changes the valve characteristic of the engine valve. By controlling the variable valve-characteristic mechanism, the engine valve opens and closes in accordance with a value of the valve characteristic appropriate for the current operating state of the internal combustion engine. The operating position of the variable valve-characteristic mechanism is detected, and the variable valve-characteristic mechanism is controlled so that a value of the valve characteristic corresponding to the operating position is equal to a target value of the valve characteristic corresponding to the operating state of the internal combustion engine.
In the valve-characteristic control apparatus, the operating position of the variable valve-characteristic mechanism detected by a sensor may deviate from the actual operating position due to differences in the output characteristic of the sensor, deviations in the positioning of the sensor, deterioration of the sensor with age, and the like. As such, it becomes difficult to open and close the engine valve in accordance with a value of the valve characteristic corresponding to the operating state of the internal combustion engine.
Accordingly, the target value of the valve characteristic is changed so that the engine valve opens and closes in accordance with a specific value of the valve characteristic. After the target value of the valve characteristic is changed, the operating position of the variable valve-characteristic mechanism is learned as a reference position for the control of the valve characteristic. For example, a target duration is changed so that an intake valve opens and closes in accordance with the shortest duration. Accordingly, the variable valve-characteristic mechanism is moved to an end of the movable range thereof. The operating position of the variable valve-characteristic mechanism at this time point is learned as the reference position. Further, Japanese Patent Application Publication No. JP-A-2002-349215 describes that the reference position is learned when the internal combustion engine is in a specific operating state (i.e., when fuel supply is stopped during deceleration).
The reference position is learned irrespective of the operation performed by the driver. When the reference position is learned, the target duration is changed from a value corresponding to the engine operating state to a value for opening and closing the intake valve in accordance with the shortest duration. By adjusting the duration of the intake valve to the new target duration, the amount of air taken into a cylinder is decreased, which decreases the output torque of the engine. However, decreases in the torque output of the engine that occur regardless of any operations performed by the driver may be disconcerting to the driver. Even if the learning process is executed only when the internal combustion engine is in a specific operating state (only when fuel supply is stopped during deceleration) as described in Japanese Patent Application Publication No. JP-A-2002-349215, a driver may still be perturbed by the effect of changes in the amount of air taken into the cylinder due to the learning process.